While it has been known for a long time that various types of tissues and organisms can withstand freezing and are even preserved by freezing, the first attempts at freezing sperm cells were made before the turn of the century. In 1897, Davenport[1][B] found that human sperm would withstand freezing. For thirty to forty years after that, little attention was paid to freezing as a possible means of preserving semen. An excellent review of the early attempts to freeze sperm has been assembled by Polge and Parkes.[2] These investigators also gave a good account of their work at the British National Institute of Medical Research in London, where in 1949 they demonstrated that glycerol would protect fowl sperm so that it would survive freezing. The next year they found that bull sperm and the sperm of several other species were protected by glycerol during freezing. During the same year, Emmens and Blackshaw[3] showed that ram and bull sperm would survive freezing. In 1951 frozen semen was used to produce a calf in England and a lamb in Australia.
[B] These numbers refer to literature citations on page 38.
The highlights in the development of frozen semen have been covered by other reviews and reports. Interested persons will find the articles of Polge and Parkes[2] and Smith[4] especially good on the early history and theoretical aspects of freezing sperm. Later progress on the freezing procedure has been reviewed and covered in a number of detailed reports.[5],[6],[7] Many items not covered in those articles have been assembled here.
One of the first considerations in freezing semen is that of deciding which semen samples are to be frozen. Since preservation of the semen--the maintenance of the potential motility and especially the fertility of the sperm--is the primary aim, some attention should be directed to the kind of semen sample that will withstand freezing. Do the initial characteristics of the sample indicate whether the sperm will withstand freezing? Does maturity of the sperm affect their freezability?
=Predicting freezability.= Estimates of semen quality in the past have been based in part on the numbers of sperm present in a fresh sample and on the percentage and rate of motility shown by the sperm. These characteristics were used to determine the relationship between the original concentration of sperm (in the fresh, undiluted sample), the percentage and rate of sperm motility in the diluted samples just prior to freezing, and the percentage and rate of sperm motility following freezing and thawing. From data collected before and after freezing and thawing 54 ejaculates, it was found that there was not a significant correlation between the number of sperm present in the original sample and the percent of motile sperm present after freezing and thawing (r = 0.03). A highly significant correlation (r = 0.45) was found, however, between the percentages before freezing and after thawing. While this correlation coefficient was highly significant, its magnitude indicates that only about one fifth of the variation in percentage of motile sperm observed after freezing was accounted for by the motility of the sperm prior to freezing.